Completed the coding of da65's SEGMENT feature.

Before this commit, we could define segment ranges; but, the disassembler wouldn't do anything with those definitions.  Now, da65 will put ".segment" directives into its output.

Fixed da65's document.
Fixed the detection of where to start and stop segments.
Clarified a segment error message.
Merge pull request #142 from greg-king5/disassemble-segment

da65's segments feature